Getting x,y,z coordinates of objects in blender API FOR MULTIPLE OBJECTS
Take this image :

bpy.context.active_object only gives a reference to the active object in the given context. This is Cube.002, with the light orange outline.
Cube is selected, but it is not the active object. (dark orange outline).
When you type bpy.context.selected_objects in the blender console you get an unordered list of all the selected objects.
&gt;&gt;&gt; bpy.context.selected_objects
[bpy.data.objects['Cube'], bpy.data.objects['Cube.002']]

In python you can iterate through a list of objects with a for loop. (Documentation) Your code, which is valid for one object, can be translated for a list to :
import bpy

for obj in bpy.context.selected_objects:
    if obj.type != 'MESH':
        continue
    # Sidenote : Use @ instead of * for element-wise multiplication in matrices          
    vcos = [ obj.matrix_world @ v.co for v in obj.data.vertices ]                     
    findCenter = lambda l: ( max(l) + min(l) ) / 2      
    x,y,z  = [ [ v[i] for v in vcos ] for i in range(3) ]      
    center = [ findCenter(axis) for axis in [x,y,z] ]
    
    print(center)

Result :